1. Field of the Invention
The present invention relates to a height adjusting device for a pickup of an optical disc player.
2. Description of Background Information
In optical disc players for playing an optical information recording disc, a focus servo mechanism is used for focusing an information reading light beam on the recording surface of the disc, so as to obtain an information reading spot. The focus servo operation is performed in such a way that an objective lens of the pickup is moved by an actuator comprising a moving coil, for example, so that its distance from the disc surface is maintained constant.
By means of this focus servo operation, an information reading spot having an appropriate shape is formed on the recording surface irrespective of the difference in the disc thickness, or the fluctuation of the disc surface. However, due to variations in the disc thickness for example, there can be a deflection of the operating position of the focus actuator in a direct-current fashion. With such a deflection of the direct-current fashion, a direct current would always flow through the focus actuator, to invite the generation of heat in the coil of the focus actuator, or further to reduce the margin of the servo loop gain.
In order to cope with such a problem, a structure shown in FIG. 1, disclosed in Japanese Patent Publication No. P53-26124, is known in which the whole system of the focus actuator is moved along an axis parallel to the axis of rotation of the turntable so that the steadily appearing excursion of the focus actuator is eliminated.
In FIG. 1, the reference numeral 71 denotes a deviation detecting capacitance, which is one of factors determining the frequency of the resonance circuit 72. The reference numerals 73, 74, and 75 respectively denote a frequency comparator, a reference frequency generator, and a power amplifier. The reference numeral 76 denotes a pickup which is illustrated as a schematic cross-sectional view, and includes a permanent magnet 77, a moving coil 78, an objective lens tube 79, an objective lens 80, and a detection electrode 81. The elements 77 through 80 together form a focus actuator. The reference numerals 82 and 83 respectively denote an information recording disc and an electric conduction surface of the disc and the deflection detecting capacitance 71 is formed between this electric conduction surface 83 and the detection electrode 81. Finally, the reference numerals 84, 85 and 86 denote a light beam, a servo amplifier, and a servo motor, respectively.
With this structure, when the disc 82 is rotated, the up-and-down fluctuation of the disc 82 is transformed to the change in the capacity of the detecting capacitance 71, and further to the change in the frequency of the resonance circuit 72. The frequency of the resonance circuit 72 is compared, in the comparator 73, with a reference frequency of the oscillator 74, and an output voltage corresponding to the frequency difference is supplied to the power amplifier 75. Thus the positioning of the objective lens is performed by supplying a drive current to the moving coil so that the position of the objective lens is kept constant. On the other hand, the output signal of the power amplifier 75 is used for controlling a drive mechanism having an integral characteristic such as the servo motor 86, through the servo amplifier 85. This drive mechanism drives the whole part of the pickup 76 or the focus actuator, and performs a control operation so that the integrated value of the output signal of the frequency comparator 73 becomes equal to 0.
In this way, the time averaging value of the current flowing through the moving coil 78 of the pickup 76 becomes equal to zero, so that the displacement of the focus actuator in the direct current fashion generated by the direct current is compensated. In this way, the focus actuator as the whole system can operate in the desired state.
However, in the case of the structure described, the delay in phase of the signal appears due to the integral characteristic which is mentioned above, the operation of the servo motor would be delayed with respect to the focus error, which in turn is added to the focus servo loop as a disturbance. In addition, due to the characteristic of the servo motor, it does not operate until when the drive current has reached a predetermined level. However, if a low level dc component is always applied to the servo motor, it will remain inoperative and will cause a problem of the generation of heat.